Methods for patterning indium tin oxide films

ABSTRACT

A method of patterning an indium tin oxide film includes the steps of forming a cap layer over the indium tin oxide film and subjecting exposed areas of the indium tin oxide film to a water plasma.

FIELD OF INVENTION

This invention relates to semiconductor fabrication. More particularly,this invention relates to methods for patterning indium tin oxide films.

BACKGROUND OF THE INVENTION

Indium tin oxide (ITO) is an electrically conductive material which,when used as a thin film (e.g., between about 100 Å to about 2200 Å inthickness), is also optically transparent. Because of thesecharacteristics, ITO is used in various applications including, but notlimited to, optical microelectromechanical systems (MEMS), flat paneldisplays, solar cells, touch screens, camera lenses, and surface heatersensors.

ITO may be formed by doping Indium oxide (In₂O₃) with tin (Sn), whichreplaces the In³⁺ atoms of the In₂O₃. Thin films of ITO may be depositedon a surface using one or more of a variety of techniques including, butnot limited to, electron beam evaporation, physical vapor deposition,sputtering, or pulsed laser deposition.

Electrically conductive and optically transparent ITO structures aretypically made by depositing a thin film of ITO on a desired substrate,forming a patterned photoresist layer on the ITO film, and etching areasof the ITO film which are exposed by the patterned photoresist layer topattern the ITO film into a desired structure.

ITO films are currently etched using dry and wet methods. One commonlyused ITO film dry etching method is reactive ion etching (RIE). The RIEmethod uses a plasma that typically comprises a major proportion ofchloroform (CHCl₃) gas, which supplies a polymer, and a minor proportionof a polymer suppressant gas such as boron trichloride (BCl₃) orchlorine (Cl₂). The ion bombardment of the BCl₃/Cl₂ mixture performs thepatterning process. The high ion ratio bombardment of the RIE process isan effective method to pattern the ITO film. The RIE process, however,produces an ITO pattern edge with an inclined or tapered edge profile,rather than a vertical edge profile, which limits critical dimensionreductions. This process control problem is due to inadequate ITO etchselectivity, wherein the ion bombardment starts to etch the edges of thephotoresist pattern, thereby causing the inclined or tapered edgeprofile of the ITO pattern.

In an effort to improve ITO etch selectivity in RIE, methane (CH₄)hydrogen (H₂) gas mixtures have been used to pattern ITO films, however,this gas mixture is potentially explosive and is therefore, unsuitablewithout relatively expensive gas exhausting equipment operatingcontinuously to remove any build-ups of this gas mixture.

Wet chemical etching is a commonly used wet etching method forpatterning ITO films. ITO films may be patterned with a hydrofluoricsolution (HF) such as HF:H₂O₂:10H₂O or more commonly with a hydrochloric(HCl) solution such as HCl:H2O. Etching rates using the HF:H₂O₂:10H₂Osolution are very high at between about 100 Å (angstroms)/second toabout 150 Å/second, and are often uncontrollable.

The HCl:H2O solution in undiluted form containing 36% HCl by volumecorresponding to a molar solution, has an etch rate of about 2500Å/minute. When patterning with the HCl solution, a certain amount of ITOvarying from about 0.5 um to about 1.5 um is often etched away fromunderneath the photoresist etch mask. Hence, the edge profile of the ITOpattern is not vertical, instead being undercut and in severe cases,thin traces of the photoresist may remain on the substrate.

The non-vertical edge profiles of the ITO pattern reduce the sharpnessand resolution of the ITO pattern, which in turn, increases theprobability of photo-alignment rejection in further processing. Inaddition, the less than sharp edge profile of the ITO pattern limitsfurther reductions in line width and critical dimension. Thus, in ITOdisplay applications where panel sizes continue to become smaller, it isdesirable to increase pixel size by reducing the space between thepixels. The inability to fabricate ITO patterns with reduced line widthand critical dimension, limits the size of the pixels in small displays.

Accordingly, an ITO patterning method is needed which allows furtherreductions in ITO pattern line width and critical dimension.

SUMMARY

A method of patterning an indium tin oxide film is disclosed herein. Themethod comprises the steps of forming a cap layer over the indium tinoxide film and subjecting exposed areas of the indium tin oxide film toa gas phase etchant comprising water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts an exemplary plasma process chamber thatmay used in the method.

FIG. 2 shows a flowchart of an embodiment of a water plasma ITOpatterning method.

FIGS. 3A-3D are cross-sectional views illustrating a substrate afterperforming certain steps of the water plasma ITO patterning method.

FIGS. 4A and 4B are cross-sectional scanning electron microscopephotographs which compare the edge profiles of thin films of ITOpatterned using a prior art RIE process (FIG. 4A) and the water plasmaITO patterning method (FIG. 4B).

FIGS. 5A-5C are scanning electron microscope photographs at differentmagnifications of a thin film of electrically conductive, opticallytransparent ITO patterned for 240 seconds using the water plasma ITOpatterning method.

FIGS. 6A-6C are scanning electron microscope photographs at differentmagnifications of a thin film of electrically conductive, opticallytransparent ITO patterned for 300 seconds using the water plasma ITOpatterning method.

FIG. 7A is test pattern used in WAT spacing testing under the controlrules of a generic IC fabrication process.

FIG. 7B is a graph showing the results of WAT spacing testing under thecontrol rules of a generic IC fabrication process.

DETAILED DESCRIPTION OF THE INVENTION

A method is disclosed for patterning an electrically conducting,optically transparent thin film of amorphous indium tin oxide (ITO) on asurface, using a cap layer (operative as a hard mask) and a plasmacomprising water as an etchant species. The areas of the ITO thin filmnot covered and protected by the cap layer react with water plasma underhigh and are removed from the surface.

The method may be performed in any suitable plasma process chamberincluding, but not limited to, a conventional resist strip chamber, aplasma etch reactor. FIG. 1 schematically depicts an exemplary plasmaprocess chamber 100 that may used in the method. The plasma processchamber includes a housing 110 that defines the plasma process chamber100. A platform 120 is provided inside the chamber 100 for mounting asubstrate. A showerhead-shape gas inlet nozzle 130 is disposed above thewafer platform 120. Reaction gases are routed into the chamber 100 via agas inlet 140, which communicates with the inlet nozzle 130. An exhaustoutlet 160 connected to a vacuum pump 170 is used to evacuate theprocess chamber 100. Electric field generating means (not shown) areused to generate an electric field in the chamber 100 of a sufficientmagnitude such that a process fluid flowing in the chamber 100, breaksdown and becomes ionized. A plasma may be initiated by releasing ordischarging free electrons inside the chamber 100 using, for example,field emission from a negatively biased electrode within the chamber100.

Referring now to FIG. 2, which shows a flowchart of an embodiment of themethod, the method commences in step 200 with a substrate 180 having adielectric layer 182 formed thereon and a thin film 184 of electricallyconductive, optically transparent amorphous ITO formed on the dielectriclayer 182. In other embodiments, the dielectric layer may be omitted sothat the thin film of ITO 184 is formed directly on the substrate 180.

The substrate 180 may comprise an optically transparent glass materialor any other suitable substrate material, depending upon theapplication. The dielectric layer 182 is optically transparent and has asufficiently high index of refraction so that it operates as ananti-reflective coating (ARC). The optical transparency of thedielectric layer 182 generally depends upon the thickness of the layer.Thicker dielectric layers provide less light scattering but reduce theoptical transparency, stress and the adhesion of the layer. The exactthickness of the dielectric layer 182 depends upon the thickness of theITO. Dielectric materials having suitable optical and mechanicalproperties include, but are not limited to, niobium oxide (Nb₂O₅),tellurium dioxide (TeO₂), tantalum oxide (Ta₂O₃), and alumina (Al₂O₃).The dielectric layer 182 may be deposited using one or more of a varietyof techniques including, but not limited to, electron beam evaporation,physical vapor deposition, sputtering, or pulsed laser deposition.

The thin ITO film 184 should be deposited to a thickness which providesthe ITO film with good electrically conductivity, i.e., less than 20ohm/square and good optical transparency, i.e., higher than about 90percent light transmission. In some embodiments, the thin film 184 ofITO may be formed to a thickness ranging between about 100 Å to about2200 Å. The thin film of ITO 184 may be deposited using one or more of avariety of techniques including, but not limited to, electron beamevaporation, physical vapor deposition, sputtering, or pulsed laserdeposition.

In step 210 a cap layer 186 is deposited on the thin film 184 of ITO, asshown in the cross-sectional view of the substrate 180 shown in FIG. 3A.In some embodiments, the cap layer 186 may comprise an oxide film, suchas SiO₂, deposited to a thickness greater than 100 Å by plasma enhancedchemical vapor deposition or any other suitable method.

In step 220 of the flowchart shown in FIG. 2, a layer 188 of photoresistis deposited on the hard mask layer 186 and patterned to expose selectedareas 186 a of the hard mask layer 186. The photoresist layer 188 may bedeposited and patterned using conventional photolithographic methods.The cross-sectional view of FIG. 3B shows the substrate 180 aftercompletion of step 220.

In step 230 of the flowchart shown in FIG. 2, the exposed areas 186 a ofthe hard mask layer 186 are removed to pattern the hard mask layer 186into a desired pattern. The exposed areas 186 a of the hard mask layer186 may be removed using conventional dry or wet etching methods. Thecross-sectional view of FIG. 3C shows the substrate 180 after completionof step 230. Upon completion of the hard mask patterning step, thepatterned photoresist layer 188 may be removed using conventionalphotoresist removal methods.

In step 240 of the flowchart shown in FIG. 2, the substrate 180 mountedon the wafer platform 120 inside the plasma process chamber 100 (FIG. 1)and a process gas 150 containing one or more chemical species isintroduced under pressure into the plasma process chamber 100, via thegas inlet 140 and inlet nozzle 130. The one or more chemical species areionized by the electric field generated within the chamber.

In some embodiments, the one or more chemical species may comprise water(H₂O) and N₂ based species. Of these species, the H₂O based species is areactive species that reacts with exposed areas 184 a of thin film 184of ITO, which are not covered by the cap layer 186. The N₂ isnon-reactive species.

The pressure (partial pressure) exerted by the process gas 150 insidethe plasma process chamber 100 before initiating a plasma is set tobetween about 0.5 Torr and about 5.0 Torr. The flow rate of the H₂Obased species of the process gas 150 is set between about 200 sccm(standard cubic centimeters per minute) and about 1500 sccm. The flowrate of the N₂ species in the process gas 150 is set between about 100sccm and about 1000 sccm. The temperature of the chamber 100 is setbetween about 200° C. and about 300° C. In a preferred embodiment, thepressure exerted by the process gas 150 is set to 2.0 Torr, the gas flowrate of the H₂O species is set to 500 sccm, the gas flow rate of the N₂species is set to 200 sccm, and the chamber temperature is 245° C.

An electric field is generated inside the chamber 100 by the electricfield generating means. In one embodiment, the electric field used toexcite the plasma may be in the microwave or RF frequency range and thepower of such a field may be about 1400 watts. Free electrons aredischarged inside the plasma process chamber 100 and travel through theprocess gas to generate a H₂O plasma 190 in the chamber 100. As the H₂Oplasma 190 stabilizes, the pressure exerted by the process gas 150inside the plasma process chamber 100 is adjusted to between about 0.5Torr and about 5.0 Torr, and preferably 2.0 Torr. The temperature of thechamber is maintained between about 200° C. and about 300° C., andpreferably 245° C.

The H₂O plasma 190 is highly etch selective to the thin film of ITOrelative to the hard mask layer 186 and the dielectric layer 182 (or thesubstrate 180 in embodiments not employing the dielectric layer 182).Consequently, as shown in the cross-sectional view of the substrate 180FIG. 3D, the H₂O plasma 190 reacts with the exposed areas 184 a of thethin film 184 of ITO to remove same without substantially reacting withthe cap layer 186 or the corresponding underlying areas 182 a ofdielectric layer 182 (or substrate 180 in embodiments not employing thedielectric layer 182). In some embodiments, the cap layer 186 isremoved. In other embodiments, the cap layer 186 may remain.

FIGS. 4A and 4B are cross-sectional scanning electron microscopephotographs which compare the edge profiles of thin films of ITOpatterned using a prior art RIE process (FIG. 4A) and the water plasmaITO patterning method described above (FIG. 4B). As can be seen, the RIEprocess produces an ITO pattern edge with an inclined or tapered edgeprofile, which limits line width and critical dimension reductions. Incontrast, the superior ITO etch selectivity of the water plasmapatterning method produces a substantially vertical edge profile whichallows for further reductions in ITO line widths and criticaldimensions.

FIGS. 5A-5C are scanning electron microscope photographs atmagnifications of 40,000×, 8000×, and 40,000× of a thin film ofelectrically conductive, optically transparent ITO patterned for 240seconds using the water plasma method. FIGS. 6A-6C are scanning electronmicroscope photographs at magnifications of 40,000×, 8000×, and 40,000×of a thin film of electrically conductive, optically transparent ITOpatterned for 300 seconds using the water plasma method. In bothexamples, the exposed areas of the thin film of ITO were completelyremoved after reaction with the water plasma.

WAT spacing testing under the control rules of a generic IC fabricationprocess, further confirmed the patterning performance of the waterplasma thin film ITO patterning method. More specifically, a thin filmof electrically conductive, optically transparent amorphous ITO waspatterned into a test pattern, as shown in FIG. 7A using the waterplasma method. The spacing result of the test pattern revealed no ITOresidue remaining between the lines of ITO and the test pattern passedthe control limits of the 1.0 um pattern design, i.e., from about 12volts to about 20 volts (VF on the x-axis) and from about 0.15 to about1 microamps (IF on the y-axis) and the long term testing value under thesame condition was 17 volts, as shown in the graph of FIG. 7B.

The thermal crystallization temperature of the thin film 184 ofamorphous ITO is slightly higher than 150° C. The growth of crystallitesdispersed in the amorphous matrix may be suppressed by increasing theamount of H₂O in the plasma, while sharply enhancing the nucleation ofthe crystallites. The amount of bonded hydrogen increases and that ofoxygen vacancies decreases at the same time, with the introduction ofinhomogeneity in the amorphous matrix. Specifically, the oxygenvacancies are effectively terminated by the —OH species generated by theadded H₂O in the plasma, which reduces the number of oxygen vacanciesand suppresses the crystal growth with the H₂O addition. After thecrystallization is completed and the thin film 184 of ITO is patterned,the remaining ITO crystallites in the thin film 184 are minimal andsmall, i.e., less than 0.1 um.

One of ordinary skill in the art will appreciate that the water plasmathin film ITO patterning method may be performed in-situ withoutadditional equipment tools. Compared with the prior art etching methods,the water plasma patterning method provides better pattern edge profilecontrol via superior ITO etch selectively. In addition, the water plasmamethod is suitable for processes which involve ITO patterning including,but not limited to, optical MEMS processes.

While the foregoing invention has been described with reference to theabove, various modifications and changes can be made without departingfrom the spirit of the invention. Accordingly, all such modificationsand changes are considered to be within the scope of the appendedclaims.

1. A method of patterning an indium tin oxide film, the methodcomprising the steps of: forming a cap layer over the indium tin oxidefilm; forming a photoresist layer over the cap layer; patterning thephotoresist layer to expose selected areas of the cap layer; removingthe exposed selected areas of the cap layer to expose selected areas ofthe indium tin oxide film;and subjecting the exposed selected areas ofthe indium tin oxide film to a plasma phase etchant comprising water. 2.The method of claim 1, wherein the cap layer comprises an oxide.
 3. Themethod of claim 1, wherein the subjecting step is performed at apressure between about 0.5 Torr and about 5.0 Torr.
 4. The method ofclaim 1, wherein the subjecting step is performed at a temperaturebetween about 200° C. and about 300° C.
 5. The method of claim 1,wherein the water is provided in a process gas at a flow rate of betweenabout 200 sccm and about 1500 sccm. 6-12. (canceled)
 13. The method ofclaim 1, wherein the plasma phase etchant comprises water plasma.